1. Field of the Invention
The present invention relates to a thin film device and a method for fabricating the same. More particularly, the invention relates to a thin film device having a transparent electrode made of conductive oxide film, for example, the film devices such as liquid crystal display devices, electro-luminescence display devices and photoelectric transducer devices, and a method for fabricating the same.
2. Description of the Related Art
Thin film devices which employ conductive oxide films as transparent electrodes include liquid crystal display devices, electro-luminescence display devices and photoelectric transducer devices. A technology which uses a transparent electrode made of a conductive oxide will be described below, taking an example of an active matrix type liquid crystal display device.
An active matrix type liquid crystal display device generally has an active matrix substrate, an opposing substrate and a liquid crystal film interposed between the two substrates. The active matrix substrate has a plurality of gate bus lines, a plurality of source bus lines arranged at right angles to the gate bus lines, and a plurality of pixel electrodes arranged within regions enclosed by these bus lines, being formed thereon. Each of the pixel electrodes receives a data signal from the source bus line via a thin film transistor (hereinafter referred to as TFT) which switches on or off in response to a scanning signal transmitted on the corresponding gate bus line. The opposing substrate has a transparent opposing electrode.
With reference to FIG. 6, a TFT of prior art will be described below.
As shown in FIG. 6, the TFT has a gate electrode 2 formed on a substrate 1, a gate insulating film 3 formed over the gate electrode 2, an amorphous silicon (a-Si) film 4 formed on he gate insulating film 3, phosphorous-doped a-Si contact films 5 formed on the a-Si film 4, barrier metal layers 7, and source/drain electrodes 8a and 8b made of aluminum (Al). The gate electrode 2, the gate insulating film 3 and the a-Si film 4 may be arranged in a reversed order. The drain electrode 8b is connected to a pixel electrode 6 made from a transparent conductive film. As the material for the transparent conductive film, indium-tin oxide (ITO) is commonly used.
A process of fabricating the TFT will be described below. First, a Cr film is deposited on the glass substrate 1 and the gate electrode 2 is formed from the Cr film in the photolithography process. Then the gate insulating film 3 made of silicon nitride (SiN.sub.x), the a-Si film 4, and the n.sup.+ -type a-Si film 5 doped with phosphorous to form ohmic contact between the source/drain electrodes 8a and 8b and the semiconductor film are successively deposited in the plasma CVD process, followed by the removal of the a-Si film 4 and the n.sup.+ -type a-Si film 5, from portions other than those where the TFTs are to be formed, by etching during the photolithography process. Indium-tin oxide (ITO) is deposited so as to form the transparent pixel electrode 6 in the photolithography process. Then a titanium (Ti) film and an aluminum (Al) film are deposited in this order, and then the barrier metal layers 7 and the source/drain electrodes 8a and 8b are formed during the photolithography process, followed by the removal of the n.sup.+ -type a-Si film 5 from above the TFT channel, thereby completing the TFT.
In the method of fabricating the TFT described above, there has been such a problem that the use of a positive photoresist in the photolithography process to form the source and drain electrodes 8a and 8b leads to the corrosion of the ITO and aluminum during the development process.
The corrosion occurs because ITO and aluminum are immersed in the developer solution while being electrically connected to each other. Specifically, because the developer solution used for the positive photoresist is an alkaline aqueous solution which dissolves aluminum, cell reaction takes place between ITO and aluminum in the developer solution, resulting in the corrosion of ITO and aluminum.
When a negative photoresist is used in the photolithography process, the corrosion does not occur. Thus solutions to the problem of avoiding corrosion have been proposed, such as the use of a negative photoresist and the use of a special solution for the developer. However, since positive photoresist is preferred to form finer patterns because the positive photoresist allows higher patterning accuracy and is easier to peel off, it complicates the fabricating process and increases the cost to use a negative photoresist for the purpose of only patterning aluminum or to use a developer solution comprising a special solution.
Corrosion may not occur in the case of wiring strips formed by using a metal other than aluminum, for example, molybdenum (Mo) or tantalum (Ta). However, these metals have higher specific resistances (i.e., higher resistivities) than aluminum, and therefore the wiring strips made of such metals must be made wider. Such wider wiring strips are not suited to the high-density integration of a thin film device.